The present invention relates to improvements in a method and an apparatus for producing synthetic resin tubes by deforming a strip of synthetic resin into a tubular shape having an axis extending longitudinally of the strip and joining both lateral surfaces of the strip together while transporting the strip in its longitudinal direction.
Conventionally, a planar strip is deformed into a tubular shape by passing the strip through a first space having a gradually reducing width or peripheral length and subsequently sending the strip into a second space having a peripheral length substantially equal to the outer peripheral length of the tube to be obtained. With the conventional method or apparatus, however, the peripheral length of the second space is invariable, whilst the transverse dimension of a strip generally varies in its longitudinal direction. Consequently, where the strip has a greater width than is specified, the strip is subjected to higher resistance while travelling through the second space axially thereof, with the result that the pulling force to pass the strip through the second space stretches the strip, thereby varying the wall thickness of the resulting tube in its longitudinal direction. The increased resistance sometimes gives rise to such an accident that the strip is ruptured or becomes incapable of passing through the second space. Conversely, where the width of strip is smaller than the specified dimension, the strip passes through the second space with its side edges unjoined with each other. Thus it becomes impossible to obtain a tube or to bring the lateral surfaces into pressing contact with each other under high pressure to form a joint of sufficient strength.
This invention intends to overcome these problems heretofore experienced in the manufacture of synthetic resin tubes.